Asynchronous transfer mode (ATM) networks carry fixed sized cells within the network irrespective of the applications being carried over ATM. At the network edge or at the end equipment, an ATM Adaptation Layer (AAL) maps the services offered by the ATM network to the services required by the application. There are a number of industry standards and proposed standards covering various AALs. In particular, "B-ISDN ATM Adaptation Layer Type 2 Specification," draft Recommendation I.363.2, November 1996, of ITU-T (herein referred to as AAL-2) provides for efficient ATM transport of small, delay-sensitive packets in such applications as packet voice systems. AAL-2 is partitioned into two sublayers, the Common Part Sublayer (CPS) and the Service Specific Convergence Sublayer (SSCS).
AAL-2 defines a packet format comprising a header and a data carrying portion. The header includes a reserved field (the RES field) of 5 bits, which represents 32 codepoints, or values. The RES field is either reserved or assigned to either the CPS or a Service Specific Convergence Function (SSCF) of the SSCS.
The CPS provides the functions of multiplexing variable length packets from multiple sources into a single ATM virtual circuit and relaying these packets to form end-to-end AAL-2 connections. That portion of the RES field assigned to the CPS is used to provide user information or signaling such as a "More" bit to indicate that the current packet is segmented (i.e., the continuation of an upper-layer frame). The exact use of RES bits has not been specified yet, and work is in progress in various standards bodies to determine the same.
In the context an SSCS system conveying packet voice, the RES field is being defined in standards groups for possible use in conveying SSCS messages including sequence number information. The latter is used by a receiver to re-build a received sequence of packets in the correct order.